Is it possible to have automatic LOD (level of detail) on large scenes?
I have two pretty detailed posts that when added together, would seem to come up with a pretty good fit for your situation.

I won't detail out those steps, but I will link to them, and summarize the workflow in a summary of logic.

Creating Distance/Rotation/Scale Based Drivers - See this post

Swapping Groups of meshes with drivers - See this post

The main rundown here, would be to drive your level of detail threshold by camera distance values, then using the same logic of swapping groups of meshes, use this threshold value to determine which LOD group to select on any given instance with this driver attached.